Optical fibers, a special case of optical waveguides, have found a large number of applications in transferring at low losses electromagnetic waves and particularly visible and infrared light over long distances. Such uses are common in fields as varied as telecommunication and laser surgery. In most of these cases the design goals for the optical fibers is to minimize optical losses of the carried beam through the optical fiber, and thus the materials used are usually glasses with minimal optical losses. When short distances are contemplated, polymers having higher optical losses are often used.
Optical fibers and waveguides are usually classified into single-mode fibers (SMF) and multimode fibers (MMF). The former support a single mode of light propagation while, in the latter, a broad spectrum of modes and wavelengths propagation can occur. Traditional optical fibers consist of a core and a cladding with respective indices of refraction n.sub.1 and n.sub.2. In order to assure complete internal reflection of the transmitted light within a fiber or a waveguide, the index of refraction of the cladding is always smaller than that of the core. This assures that all light within the acceptance angle of the fiber is internally reflected at the core/cladding interface, see Handbook of Fiber Optics-theory and Applications, by Chai Yeh, published by Academic Press, San Diego, CA, 1990.
Numerous applications of optical fibers bundles to illumination are known. In most cases the fiber bundle is simply used to conduct the light to the remote location and the light is emitted from the open end of these fibers. In some instances, it is desirable to conduct electromagnetic waves along a single guide and extract light along a given length of the guide's distal end rather than only at the guide's terminating face. This special need has been recognized in the prior art and numerous approaches to the extraction of light at intervals from optical waveguides or optical fibers have been proposed. Each of these proposals, however, has its specific shortcomings making the application impractical or limited to only few situations.
For instance, Orcutt in U.S. Pat. No. 4,422,719, proposes the extraction of light from a light guide by enclosing the waveguide within a transparent sleeve having an index of refraction greater than the index of refraction of the waveguide and embedding within the sleeve light-reflecting powders, or by providing other discontinuities such as cuts or air bubbles within the fiber core. This approach has a number of shortcomings. First, the light extraction rate along the guide declines monotonically (and quite rapidly) from the proximal end to the distal end. The higher index of refraction of the cladding causes conversion of core modes (light propagation mode) to cladding modes to occur at the proximal end or the composite guide, thus sharply depleting the beam intensity as the light traverses the full length of the guide. Furthermore, the use of particles and bubbles suspended within the cladding causes excessive absorption of the light in the transmitting medium (particularly the cladding itself). Orcutt attempts to overcome the lack of light extraction control by including in the core refracting discontinuities or "light extraction" cuts through the cladding to the core and spacing these as a function of the distance from the light source. This approach is difficult to implement and furthermore, creates a series of discrete light sources along the guide and does not allow for continuous light extraction.
Mori (U.S. Pat. Nos. 4,460,940, 4,471,412 and 4,822,123) uses discrete light diffusing elements on a light transmission element to extract light from said light guide. In U.S. Pat. No. 4,460,940, Mori uses convex or concave diffussing elements to extract light of a specific wavelength, and a set of discrete elements with increasing density (but constant thickness) toward the distal end of the transmitting medium to extract light (presumably all wavelengths) from the transmitting element.
In U.S. Pat. Nos. 4,471,412 and 4,822,123, Mori uses discrete light outlets on a light conducting member. In the former patent he uses discrete diffusing elements without consideration to their quantitative light extraction capabilities while in U.S. Pat. No. 4,822,123 he uses light scattering discrete elements and simply increases their number as he approaches the distal end of the light conductor. The disadvantages of Mori's light extraction systems include discontinuity of the light sources in that the appearance of the device includes a plurality of concentrated light sources, and the great difficulty in correctly spacing and sizing the extraction elements to provide for controlled light extraction from the light guide. Furthermore, the manufacturing and assembly of the devices of Mori is awkward and costly.
Cheslak U.S. Pat. No. 4,765,701 also uses discrete elements to extract light from an optical fiber in conjunction with a panel. Cheslak uses angular recesses and does not provide for means to control quantitatively the light extraction, and as a result, the illumination from the downstream (distal) recesses is progressively lower.
The prior art as described is thus wanting in the areas of controlled light extraction from optical fibers and waveguides and to the extent that a minimal control is gained, the prior art provides light output that occurs in discrete segments rather than in a continuous manner. Furthermore, some of the prior art fails to provide for completion of light extraction along the transmitting medium, leaving an undetermined portion of the light transmitted through the waveguide to be emitted, presumably at the distal end of the guide.